Birds are the only dinosaurs that have survived into modern times. Why is that? Of all the dinosaur species, how did they manage to make it through the catastrophic events of 65 million years ago, whereas all their fellow dinos perished? A new study, published May 27 in Nature, hints at an evolutionary phenomenon that may have played to birds' advantage: They are, it seems, baby dinosaurs whose biology prevents them from ever growing up. (Scientific American is part of Nature Publishing Group.)
Bhart-Anjan Bhullar and his doctoral advisor, Arkhat Abzhanov, posit that birds may have evolved from dinosaurs by a process known as paedomorphosis, whereby an organism retains juvenile traits even after it becomes sexually mature. "They certainly look to have some paedomorphic characteristics," says Jack Horner, a Montana State University paleontologist who studies dinosaur growth and development and was not involved in the study. Paedomorphosis "is not uncommon in evolution and speciation," he adds.
Bhullar and Abzhanov reached this conclusion by comparing the skulls of birds and dinosaurs across phylogenies, or related groups, and at different developmental stages. To quantitatively compare cranial geometries, they scanned the skulls of theropod dinosaurs (which are thought to be birds' ancestors), crocodiles and alligators (dinosaurs' cousins), early transitional birds such as Archaeopteryx, and modern birds. Then they created digitized versions of each skull and mapped out cranial landmarks, such as nostril tips, eye socket dimensions and places where bones meet.
Their measurements showed that whereas a typical non-avian dinosaur began life with a round head, large eyes and a big brain (relative to the rest of the body) then later developed an elongated snout and smaller relative brain size, birds kept their baby faces.
If birds did evolve by paedomorphosis, they join species such as axolotls. These salamanders evolved to retain tadpolelike gills and fins and, unlike most other amphibians, remain aquatic into adulthood. This feature appears to be due to a hormone disruption. By adding thyroid hormone into their water, researchers have caused axolotls to metamorphose into terrestrial salamanders.
But why would it be advantageous for adult animals to look like kids? Greg Erickson, who studies evolutionary morphology at The Florida State University and was not involved in the study, says that paedomorphosis can help a species to develop new adaptations and exploit new niches. In particular, he suggests that paedomorphosis may have enabled birds to develop larger eyes, which aid in spatial assessment during flight as well as a high brain-to-body-mass ratio, which may contribute to intelligence.
An even simpler explanation is that kids are small and, in times of environmental stress, small is good. Bhullar cites an example of Temnospondyli—large primitive amphibians that were common before 120 million years ago. Catastrophic events killed off most of the temnospondyli, except for a few paedomorphic species. "The interesting thing to me is that [after the catastrophe] these little paedomorphic animals were at the base of a giant radiation," Bhullar says. He suggests that a similar phenomenon may have occurred during the catastrophic events that killed the dinosaurs—being small may have been an advantage, because smaller animals require less food and can more easily hide. "Everything that lived on land and weighed more than one kilogram perished," Bhullar says. "The only dinosaurs that survived were the paedomorphic ones." And after many of those larger species went extinct, the little dino may have been better placed to exploit the new niches that opened up.
Paedomorphosis "may have been the linchpin that helped the birds to diversify," Erickson agrees.
To this day, birds' smaller size suits them. With 10,000 different species, they are one of the most diverse groups of land-dwelling animals.